In this article, we explore the application of Scanning Capacitance Microscopy (SCM) for studying focused ion beam (FIB) induced damage in silicon. We qualitatively determine the technologically important beam shape by measuring the SCM image of FIB processed implantation spots and by comparison of topographical and SCM data. Further, we investigate the question how deep impinging ions generate measurable damage below the silicon surface. For this purpose, trenches were manufactured using FIB and analyzed by SCM in cross sectional geometry

A Lugstein

E Bertagnolli

E Gornik

H Wanzenböck

J Smoliner

W Brezna

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Proceedings GMe Forum 2003 137 
Scanning Capacitance Microscopy 
Investigations of Focused Ion Beam 
Damage in Silicon 
W. Brezna, H. Wanzenböck, A. Lugstein, E. Bertagnolli, E. Gornik, 
J. Smoliner 
Institute for Solid State Electronics, Vienna University of Technology 
In this article, we explore the application of Scanning Capacitance Microscopy (SCM) 
for studying focused ion beam (FIB) induced damage in silicon. We qualitatively de-
termine the technologically important beam shape by measuring the SCM image of 
FIB processed implantation spots and by comparison of topographical and SCM 
data. Further, we investigate the question how deep impinging ions generate meas-
urable damage below the silicon surface. For this purpose, trenches were manufac-
tured using FIB and analyzed by SCM in cross sectional geometry. 
Introduction 
Focused ion beam (FIB) techniques are among the most important tools for modifica-
tion tasks below 100 nanometer. Today, FIB systems are mainly used for device modi-
fication, transmission electron microscopy (TEM) sample preparation, scanning probe 
microscopy (SPM) tip preparation, and deposition of different metals and insulators [1]. 
Unfortunately, there are mainly two effects that limit the usage of FIB modification to 
certain applications and areal scales [2]. Difficult to measure, the ion beam diameter 
and intensity profile defines the lateral resolution and the smallest possible size of a 
FIB made structure. The other limitation occurs due to ion beam induced damage, 
which extends far below the modified sample surface. 
Various methods such as secondary ion mass spectroscopy (SIMS) and transmission 
electron microscopy (TEM) have been utilized to measure penetration depths and in-
tensity profiles of FIBs. However, the disadvantage of these methods is their lack of 2D 
spatial resolution (SIMS) or difficult sample preparation (TEM). For this reason, scan-
ning probe techniques have successfully been applied for FIB intensity profile determi-
nation and imaging in other ion beam applications [3]. Topographic atomic force mi-
croscopy (AFM) investigations can yield high resolution data from FIB implanted spots 
via the embossment of FIB amorphisized areas due to the slightly lower density of 
amorphous silicon compared to crystalline silicon. However, severe degradation of the 
electronic properties will occur long before the structural changes (amorphization) take 
place, which cannot be investigated any more with topographic AFM. In this article, we 
introduce Scanning Capacitance Microscopy (SCM) as a very sensitive tool to investi-
gate the position and extent of FIB induced electronic degradation. SCM is an exten-
sion of conventional AFM and a very promising tool for semiconductor device charac-
terization. The current state of the art of this technique can be found in the review arti-
cle [4]. SCM can detect magnitudes smaller changes in material composition than any 
other scanning probe method and it is possible to sense as small quantities as 10 – 
100 impurity atoms per cubic micron (1013 – 1014 per cm3). Where FIB irradiation does 
not alter the topography of the region of interest, it is still possible to get reliable data of 
the FIB damaged areas via SCM. We first use this method to determine the ion beam 
intensity profile by investigating irradiated silicon surfaces. Then we will deal with the 
damage spread inside the silicon sample beneath a FIB milled trench. 
138 W. Brezna et al. 
Experimental 
To avoid any additional difficulties in data interpretation due to the very complex elec-
trical behavior of pn-junctions [5] we decided to use low p-doped silicon wafers, since 
our FIB system is equipped with a Ga+ ion source (acceptors). The low acceptor 
concentration of the bulk material was an advantage, because SCM yields higher 
signals on low-doped semiconductors. The samples were prepared in two ways to 
match the different demands of FIB intensity profile and damage depth determination: 
For measuring the beam intensity profile, we used the very clean surfaces of freshly 
cleaved silicon wafers. On the cleaved surface five types of spots were made with the 
FIB system, which differed from each other in the deposited ion dose (0.025, 0.05, 0.1, 
0.5 and 5 pC/spot). The spots were located in close vicinity (a few microns) of the 
wafer edge to reduce sample tip-holder overlaps and related stray capacitance, which 
increases the signal to noise ratio in our SCM measurements. The acceleration voltage 
of the Ga+ ions was 50 kV and the aperture size was 50 µm. For the investigation of the 
damage depths, a trench was milled into the polished front side of a wafer. After 
milling, the sample was cleaved to get cross sections of the damaged area beneath the 
trench. 
Results and Discussion 
Although the SCM signal is a complicated function of the semiconductor doping con-
centration and the applied tip-bias voltage, it is possible to adjust the electrical parame-
ters of the setup in a way to gain a signal that is monotonic with doping concentration 
[6]. Using such optimized conditions leads to good SCM contrast with big signal in low-
doped regions and small signal in higher doped regions. However, besides the p-type 
doping by Ga+ ion implantation, the crystalline structure of FIB irradiated samples is 
damaged heavily which also results in a reduction of the SCM signal. Therefore, we 
cannot distinguish between high doped or damaged areas. Studies show that ion beam 
doses magnitudes smaller than the minimum dose for surface modification (e.g. swell-
ing due to amorphization) can already be detected via SCM. We used this fact to de-
termine the beam intensity profile of our FIB system. Figure 1 (a) shows the resulting 
topographic changes of milling with a moderate dose per spot (0.5 pC/spot). The swell-
ing and subsequent silicon removal due to sputtering can be seen very well and leads 
to the typical crater-like structures. Figure 1 (b) shows the simultaneously measured 
SCM picture. In comparison with the topographic image seen in Fig. 1 (a), the recorded 
SCM picture shows a significantly larger damaged region indicated by the dark circle of 
low SCM signal. Based on topography, we define the ion beam radius as follows: the 
radius RTopo is the distance from the deepest milled point (highest intensity in the beam 
center) to the point where the outer swelling flank is half-decayed. In the case where 
the ion dose is so small that only swelling is observed, we take the distance between 
the maximum of the swelling and the point where it is half-decayed.  Based on SCM we 
defined the beam radius RSCM as half the distance between the points where the SCM-
signal-flanks rise to half of their maximum. 
The two radii RTopo and RSCM are compared in Fig. 1 (c), where cross sections of the 
image data are plotted along line L. The difference between topography signal and 
SCM signal, ∆R = RSCM - RTopo, is 250 nm. Figure 1 (d) compares the behavior of the 
radii RTopo and RSCM with increasing dose per spot. The radii show monotonic growth, 
however, there are saturation effects in the high dose regime. In addition, both data 
sets diverge for big ion doses. Whereas for the lowest dose the structures have twice 
the radius in SCM mode than in topography mode, for the highest dose this ratio is 
almost four. Our observation that the SCM based beam radius RSCM is always larger 
than the topographic radius RTopo, and the effect that RTopo and RSCM diverge for big ion 
doses, can be explained by the following facts. First, as was already published [1], [7], 
the beam profile consists of (at least) two regions: The region far away from the beam 
SCM Investigations of Focused Ion Beam Damage in Silicon  139 
center, where the overall intensity is very small but decays slowly. The other region is 
close to the beam center, where the intensity is comparable with the beam center and 
has the steepest decay. Second, SCM is much more sensitive to ion irradiation effects 
than the topographic signal, since topographic changes by amorphization need very 
high ion doses. Because of these two properties, the SCM sensed radius grows quickly 
with increasing dose per spot in the outer areas of the beam profile, whereas the 
smaller crater-like structure in the topography grows just slowly. 
 
Fig. 1: (a) The topographic image of a FIB irradiated spot with an inset showing a 
magnification. The beam direction is indicated. (b) Corresponding SCM image. 
(c) Radial cross sections through the topographic and SCM images along the 
line L. RTopo and RSCM define the radius of damage as seen in the topographic 
and the SCM image. (d) RTopo and RSCM versus ion dose D. 
A second important subject for FIB application is the determination of the width and 
depth of FIB induced damage below the sample surface. Figure 2 (a) shows the topog-
raphy of the cleaved, FIB made trench. The corresponding SCM signal is shown in Fig. 
2 (b). Again, we optimized the SCM bias to obtain the largest SCM signals in the unim-
planted, low-doped areas. Figure 2 (c) compares the topographic and the SCM signal 
height plotted along line L|| parallel to the incident beam. The distance between the 
trenches side walls and the SCM signal ∆R|| is approximately 620 nm. Previous TEM 
investigations of deep FIB milled polysilicon show an amorphisized region that extends 
about 200 nm in depth [8], which is only a third of the distance we have measured. 
This can be explained by comparing the detection sensitivity of TEM and SCM. To get 
contrast in TEM, a crystalline substrate has to be amorphizised to a high extent, which 
needs relatively high ion doses. On the other hand, SCM is able to detect impurity con-
centrations down to 10 – 100 atoms per µm3 (1013 - 1014 per cm3), which is magnitudes 
more sensitive than TEM. Figure 2 (d) shows plots of the topographic and the SCM 
signal heights along line L perpendicular to the FIB direction. With a certain probability, 
ions can be scattered out of their incident direction. In this way, they can reach areas 
not covered by the beam area. The distance between the trenches sidewall and the 
SCM signal ∆R⊥ is about 310 nm. A comparison between Fig. 2 (d) and Fig. 2 (c) 
shows a ratio ∆R⊥/∆R|| of 1/2, which is confirmed by previous TEM investigations of FIB 
induced  damage in polysilicon-gates of MOSFETs [8]. 
140 W. Brezna et al. 
 
Fig. 2: (a) The topographic image of a FIB milled trench in cross sectional view. The 
beam direction is indicated. (b) Simultaneously recorded SCM image. (c) Com-
parison of topographic and SCM signal heights along the line L|| parallel to the 
ion beam. (d) Topographic and SCM signal heights along the line L⊥ perpen-
dicular to the incident beam. 
Conclusion 
In summary, we demonstrated the utilization of SCM for the characterization of FIB 
processed samples. First, the beam shape of a FIB machine was determined by taking 
SCM images of spots created by FIB irradiation. Our data indicate a beam shape con-
sisting of a high intensity central region, which decays very quickly when the center is 
left, and vast tails where the intensity is small but declines more slowly with radial dis-
tance. Second, we investigated how deep below the FIB modified surface changes in 
the structural and electrical properties can be sensed. Damage was detected in much 
greater depths than reported by other authors, which is probably due to the increased 
sensitivity of SCM compared to other methods. Finally, the damage depth in the direc-
tion parallel to the incident beam and perpendicular to it has been investigated and was 
found to be in agreement with literature. 
Acknowledgements 
This work was sponsored by “Fonds für innovative Projekte an der TU Wien”, “Gesell-
schaft für Mikroelektronik (GMe)” and “Fonds zur Förderung der wissenschaftlichen 
Forschung (FWF)” project No Z24-TPH. 
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Scanning Capacitance Microscopy Investigations of Focused Ion Beam Damage in Silicon

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Scanning Capacitance Microscopy Investigations of Focused Ion Beam Damage in Silicon

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